The invention relates to an anisotropic bonding system and process, and more particularly to an anisotropic bonding system and process that monitors an electrical characteristic of the bond in real-time to determine bond quality.
Anisotropic conductive films (ACF) and anisotropic conductive pastes (ACP) are becoming commonly used in the electronics industry to connect components together. These pastes or films 100 are composed of electrically conductive particles 100 dispersed in a non-conductive carrier material 104, such as a thermoset material, a thermo-setting material or a blend of the two, as illustrated in FIG. 1. Anisotropic bonding materials are commonly used to electrically connect the surfaces of conductors on two surfaces while at the same time isolating the connected conductors from adjacent conductors. This is due to the conductive properties of the anisotropic bonding material; when applied, the material conducts in the Z-axis direction only and not in the X- or Y-axes.
To join two conductive surfaces together, the anisotropic paste or strip material is placed between the two surfaces to be joined, and heat and pressure is applied to the assembly to bring the conductive surfaces into contact with the conductive particles in the film or paste, thereby forming an electrical path between the two conductive surfaces. The anisotropic material is available with various conductive particle sizes and distribution densities within the carrier material so that the appropriate anisotropic material can be selected for a given application. Generally, materials having smaller conductive particles are selected for conductors having finer pitches to avoid short circuits in the X or Y directions.
To improve the connection between the conductive surfaces being joined, there has been a trend toward using anisotropic materials with compressible conductive particles, with conductive metal-coated plastic particles being the most common. These particles are able to conduct electricity through the outer metal plating and are also compressible and have memory retention. The compressible conductive particles, however, require careful control over the compression process to avoid over- or under-compression, as illustrated in FIGS. 2 and 3.
More particularly, if the assembly is not compressed enough, as shown in FIG. 2, there will be an insufficient number of particles 102 that contact each other and the conductive surfaces to form a continuous current path, creating an interconnect exhibiting a high resistance. Further, if the particles 102 are not sufficiently compressed, the thermal cycling range of the assembly will be greatly reduced for reliable operation.
If the assembly is compressed past the elastic limit of the particles 102 as shown in FIG. 3, however, the metal coating on the particles 102 will form cracks 106 that interrupt the electrical path and increase the resistance in the interconnect as well. Further, cracks 106 in the particles 102 reduce the thermal cycling range of the assembly because thermal cycling may increase crack propagation and because the particles 102 are deformed beyond their elastic limits.
Current bonding methods adjust heat, time, and pressure of the assembly system to attempt to establish an acceptable bond. These methods tend to slightly over-cure the material and apply slightly too much pressure to ensure reliable bonding and to compensate for variations in the anisotropic material and temperature drift. The curing schedules of anisotropic bonds depend on time and temperature and have a relationship generally as follows:
T≅30/2{circumflex over (0)}((Te-180)/10)
Te≅(xe2x88x9210*(ln(T)xe2x88x92ln(15)xe2x88x9219* ln(2)))/ln(2)
where T=Time in seconds.
Te=Temperature in degrees Centigrade at the bond line and where in is a natural logarithm.
In this relationship, however, a 10-degree difference in bond line temperature can affect the bonding cure time significantly, by a factor of ≅xe2x88x9250%, +100%, depending on the direction of the temperature variation.
Further, known bonding processes apply temperature and pressure to the bond according to predetermined parameters without referring to the bond""s actual characteristics during the compression process. FIG. 4 is a graph illustrating a bonding cycle that allows over-compression of the bond; as can be seen in the FIGURE, the pressure 400 is kept constant throughout the temperature cure schedule of this example. As can be seen in the graph, the high initial bond resistance 400 (due to the insulating characteristics of the carrier material) quickly decreases between 0 and 20 seconds as electrical connections start to form. At 11 seconds, the conductive particles start to form a path between the surfaces being connected 402 drops significantly between 20 and 24 seconds. The resistance falls further between 11 and 19 seconds, at which the traces are completely connected by conductive particles in the anisotropic material. The bond resistance stabilizes at around 24-30 seconds, but, as can be seen in the graph, pressure 400 continues to be applied to the bond at a constant rate.
At 36-40 seconds, the bond resistance 402 begins rising undesirably due to cracking of the electrically conductive coating on some of the compressed particles. After 40 seconds, the bond resistance stabilizes at the increased level because further compression is not possible at the selected temperature/pressure settings. Because known bonding processes keep the pressure level constant throughout the curing process, there is no way to prevent over-compression if cracking begins to occur before the temperature cure cycle is complete.
There is a need for a bonding process that reliably produces conductive bonds using anisotropic material without experiencing any increase in bond resistance or damage to conductive particles in the material.
Accordingly, the present invention is an anisotropic bonding system and method that prevents over-compression of the bond by monitoring an electrical characteristic, such as resistance, of the bond in real time during the compression and curing process. If the bond resistance reaches a predetermined level before the temperature cure cycle is over, the system reduces the pressure applied to the bond to a holding/clamping value as the curing process for the carrier portion of the anisotropic material completes. Because the bond characteristics are monitored during the bonding process, the conductive particles stay within their elastic limits, thereby minimizing or preventing cracking in the conductive coating, to ensure that the bond resistance remains low.